The present invention relates to apparatus for indicating and displaying the focus of an objective, in particular that of a photographic camera.
It is known to the prior art, as regards focusing of objectives, to project an image of the object onto a scanning disk or plate located in one image plane of the objective. This disk or plate comprises a grating being moved normal to its grating lines. The light rays impinging on this grating are split into two separate light beams which originate from different areas of the exit pupil of the objective. Two photoelectric detectors are provided behind the scanning disk or plate, each of which delivers one electrical signal, the phase difference between the two signals representing a measure of the focus or lack of focus of the objective.
It is known to use a phase difference signal to control a servo motor displacing the objective by means of gearing into one direction or the other until the phase difference is null, that is, until the objective is focused on the object.
However, the objectives to be displaced frequently are fairly heavy, so that their displacement requires significant power and a strong motor is required, which in turn often is packaged only with difficulty in optical instruments. This is especially the case for camera objectives having long focal lengths.
The state of the art of scanning disks and gratings in combination with photoelectric detectors therebehind for measuring the focus of objectives may be ascertained by reference to U.S. Pat. Nos. 3,781,110 and 3,833,299 the disclosures of which are incorporated herein. The state of the art of the circuitry for auotomatic comparison of two beam photoelectric radiation may be ascertained by reference to U.S. Pat. No. 3,255,355, the disclosure of which is incorporated herein.
Having the above limitations in mind, it has been proposed therefore to make use of a simple control in lieu of a closed loop, that is, to provide semi-automatic focus instead of a fully automatic one, the objective being adjusted manually and a special focus indicator being provided.
Conceivably such a focusing display might be a simple measuring unit requiring displacing the objective until a given pointer position is reached. Such a measurement unit however will be disadvantageous if the light conditions for the object are poor, that is, when the object is fairly dark, because there would be difficulties in reading the pointer position.